Unpredictablity of PCR product

Hi. I was wondering why sometimes I seem to get PCR product using the same protocol and other times nothing shows up. The DNA that I use is a linear plasmid DNA cut with MssI and has been in storage for 2-3 years. I was wondering if there is any other factor other than degradation? I vortex and spin down the DNA. I am not sure what is wrong. Thanks.

Hey, vortexing is not a good idea, because it breaks DNA, centrifugation shouldn't be a problem...

Thank you. I was wondering if Plasmid DNA could also be affected. The reason I asked was the DNA I am using is 2-3 years old. Should I be extra careful and just flick it before adding to my PCR tube? Thank you.